A communication system is formed, at a minimum, by a transmitter and a receiver interconnected by a communication channel. The transmitter forms at least a portion of a sending station, and the receiver forms at least a portion of a receiving station. Communication signals transmitted by the sending station are transmitted upon the communication channel to be received by the receiving station.
A radio communication system is a communication system in which the communication channel is formed of a portion of the electromagnetic frequency spectrum. A fixed or hard-wired connection is not required to form a link between the sending and receiving stations. A radio communication system is therefore advantageously utilized when the use of such a fixed or hard-wired connection would be inconvenient or impractical.
One type of packet data radio communication system communicates packet data between the sending and receiving stations. In packet data communications, data which is to be sent to the receiving station is formatted into packets. Once formatted into packets, the information can be transmitted to the receiving station in discrete bursts, formed of a single packet, or a single series of packets, to the receiving station. In several of such systems, the packets are transmitted when a channel is available upon which to transmit the packet.
A Mobitex (tm) system is exemplary of a radio communication system which utilizes packet data communications to communicate information between a sending station and a receiving station. A Mobitex (tm) system is a multi-user system formed of a Mobitex network and a plurality of mobile radio modems. Information to be communicated to a mobile radio modem is formatted into frames of data, each frame of data forming a packet which can be transmitted to the mobile radio modem. Sequences of frames are transmitted to the mobile radio modem to effectuate the communication of significant amounts of information to the mobile radio modem. When the mobile radio modem receives a message frame, the modem transmits, back to the Mobitex network, an acknowledgment signal, acknowledging reception of the message frame.
Because a radio communication channel is not an ideal channel, the message frames transmitted upon such channels are susceptible to signal quality degradation. Such signal quality degradation can occur, for example, due to interference introduced upon the signal channel. excessive separation distance between the Mobitex network and the mobile modem, or other communication vagaries. If the signal quality degradation is significant, a message frame transmitted by the Mobitex network might not be detected by the mobile radio modem.
If the mobile radio modem does not detect the frames, no acknowledgment is returned to the network. In spite of such lack of acknowledgment, in at least one existing Mobitex system, a large number of message frames, up to, or in excess of, twelve message frames, are continued to be transmitted to the mobile radio modem, without acknowledgment by the mobile radio modem of any of the message frames. And, even after transmission of such large number of message frames, the network merely designates the radio modem to be in an inactive state. The sender of the message frames might then be able merely to store such message frames, otherwise to be transmitted to the radio modem, in a memory location, such as a message mailbox location associated with the radio modem.
As with many other radio communication systems, the portion of the electromagnetic spectrum allocated for use by a Mobitex system is limited. Any manner by which the allocated portion of the spectrum can be more efficiently utilized would permit the capacity of the Mobitex system to be effectively increased. Because the transmission of the long sequences of message frames to a mobile radio modem, unable to receive such frames, is an inefficient utilization of the allocated spectrum portion, a manner by which to determine quickly such lack of reception of the message frames would be beneficial.
More generally, in any communication system in which packet data is communicated, any manner by which to more efficiently utilize the channel capacity of channels allocated in such a system would be advantageous.
It is in light of this background information related to communication systems utilizing packet data communications that the significant improvements of the present invention have evolved.